About pandemics
Influenza pandemics
An influenza pandemic is an outbreak of influenza in many countries at once, caused by a new virus that human beings have no immunity to, and that spreads very easily from one person to another.
Based on historical patterns, influenza pandemics can be expected to occur, on average, three to four times each century, when new virus subtypes emerge and are readily transmitted from person to person.
But the actual occurance of influenza pandemics is unpredictable. Although nobody can say when one might evolve, experts agree that another influenza pandemic is inevitable.
The World Health Organisation will determine whether the virus is spreading sufficiently to constitute a pandemic, at which point the New Zealand Government would lead a national response.
Swine influenza
Swine influenza or 'swine flu' is a highly contagious acute respiratory disease of pigs, caused by one of several swine influenza A viruses. These are most commonly of the H1N1 subtype, but other subtypes are circulating in pigs (H1N2, H3N1, H3N2). Sometimes pigs can be infected with more that one virus at a time, which can allow the genes from these viruses to mix. Although swine influenza viruses are normally species specific and only infect pigs, they sometimes cross the species barrier to cause disease in humans.
(source: World Health Organisation website)
Avian influenza
Avian influenza is an infectious disease that doesn't normally infect species other than birds and pigs, but has, in some parts of the world, been transmitted from birds to humans.
The first documented infection of humans with an avian infuenza virus occurred in Hong Kong in 1997, when the H5N1 strain caused severe respiratory disease in 18 humans, six of whom died.
If more people become infected over time, the likelihood also increases that humans, if they are infected with human and avian influenza strains at the same time, could serve as a 'mixing vessel' to create a new strain of the virus, which could be transmitted from person to person.
Such an event would mark the start of an influenza pandemic.